Circuit for cancelling and replacing redundant elements

ABSTRACT

In an integrated circuit having addressable primary elements and redundant elements which can be programmed to replace primary elements, a circuit and method are provided for cancelling and replacing redundant elements. A circuit is described which can be used in a memory such as a dynamic random access memory (DRAM) which uses a selectively blowable anti-fuse to disable a redundant element which was previously programmed to replace a defective primary element. The disclosure describes a method for permanently cancelling the defective redundant element and replacing the defective redundant element with another redundant element.

This is a continuation of application Ser. No. 08/417,007, filed Apr. 5,1995 abandoned May 29, 1997.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and inparticular the present invention relates to cancelling and replacingredundant elements in integrated circuits.

BACKGROUND OF THE INVENTION

As the number of electronic elements contained on semiconductorintegrated circuits continues to increase, the problems of reducing andeliminating defects in the elements becomes more difficult. To achievehigher population capacities, circuit designers strive to reduce thesize of the individual elements to maximize available die real estate.The reduced size, however, makes these elements increasingly susceptibleto defects caused by material impurities during fabrication. Thesedefects can be identified upon completion of the integrated circuitfabrication by testing procedures, either at the semiconductor chiplevel or after complete packaging. Scrapping or discarding defectivecircuits is economically undesirable, particularly if only a smallnumber of elements are actually defective.

Relying on zero defects in the fabrication of integrated circuits is anunrealistic option. To reduce the amount of semiconductor scrap,therefore, redundant elements are provided on the circuit. If a primaryelement is determined to be defective, a redundant element can besubstituted for the defective element. Substantial reductions in scrapcan be achieved by using redundant elements.

One type of integrated circuit device which uses redundant elements iselectronic memory. Typical memory circuits comprise millions ofequivalent memory cells arranged in addressable rows and columns. Byproviding redundant elements, either as rows or columns, defectiveprimary rows or columns can be replaced. Thus, using redundant elementsreduces scrap without substantially increasing the cost of the memorycircuit.

Because the individual primary elements of a memory are separatelyaddressable, replacing a defective element typically comprises blowingfuse-type circuits to `program` a redundant element to respond to theaddress of the defective element. This process is very effective forpermanently replacing defective primary elements. A problem with thisprocess is the possibility of replacing a defective primary element witha defective redundant element. The possibility of having a defectiveredundant element increases as the number of redundant elements on anintegrated circuit increases. Because the process of replacing defectiveelements is a permanent solution, if a defective redundant element isused, the circuit must be scrapped.

The number of redundant elements provided on a circuit usually exceedsthe number of redundant elements needed to `repair` a defective chip,therefore it would be desirable to replace the defective redundantelement with another available redundant element.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora circuit and method for cancelling and replacing defective redundantelectronic elements on an integrated circuit.

SUMMARY OF THE INVENTION

The above mentioned problems with repairing defective redundant elementsand other problems are addressed by the present invention and which willbe understood by reading and studying the following specification. Acircuit and method are described which provide for cancelling andreplacing defective redundant elements in an integrated circuit,electronic memory or other electronic device.

In particular, the present invention describes an integrated circuithaving a plurality of selectively addressable primary elements and aplurality of selectively programmable redundant elements. The integratedcircuit comprises an enable circuit used for enabling one of theredundant elements, a program circuit for selectively programming theenabled redundant element. The programmed redundant element, therefore,can be used to replace a defective primary element. The integratedcircuit also has a non-volatile disable circuit for disabling theenabled redundant elements.

In another embodiment, an integrated circuit memory array is described.The memory array has a plurality of selectively addressable primarymemory elements and a plurality of selectively programmable redundantmemory elements. The integrated circuit memory comprises an enablecircuit for enabling one of the redundant memory elements, a programcircuit for selectively programming the enabled redundant memory elementto replace a defective primary memory element, and a non-volatiledisable circuit for disabling the enabled redundant memory element.

In another embodiment, the enable circuit comprises a blowable fuse. Oneembodiment of the disable circuit comprises a latch circuit forindicating when an enabled redundant memory element is addressed, and afuse circuit for producing an output used to disable the enabledredundant memory element. The fuse circuit can comprise a selectivelyblowable anti-fuse. The fuse circuit can alternatively comprise a highvoltage P-channel transistor having its drain connected to a low voltageline, and an anti-fuse electrically connected between a variable voltagesource and a source of the high voltage P-channel transistor.

Still another embodiment of the invention provides a method of repairinga selectively addressable defective primary element in an integratedcircuit. The method comprises the steps of programming a first redundantelement to respond to an address of the defective primary element andenabling the first redundant element. If the first redundant element isdefective, the first redundant element is disabled and a secondredundant element is programmed and enabled to respond to an address ofthe defective primary element. A method of cancelling a defectiveredundant element is also provided. To cancel a defective element, thedefective redundant element is addresses and an anti-fuse circuit isselectively blown to permanently disable the redundant element.

A final embodiment provides a method of modifying an integrated circuithaving a plurality of redundant elements. The method comprises the stepof permanently replacing one redundant element with a second redundantelement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a match fuse bank associated with a redundant element;

FIG. 2 is a block diagram of one embodiment of the present invention;

FIG. 3 is a schematic diagram of the latch match circuit and cancel fusecircuit of FIG. 2;

FIG. 4 is a redundant match fuse circuit incorporating features of oneembodiment of the present invention;

FIG. 5 is a schematic diagram of the fuse bank enable circuit of FIG. 2;and

FIG. 6 is a timing diagram of the circuit of FIG. 2 being used to cancela redundant element.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent inventions. The following detailed description is, therefore,not to be taken in a limiting sense, and the scope of the presentinventions is defined only by the appended claims.

The present invention relates to integrated circuits such as memorycircuits. In particular the present invention provides a circuit andmethod for cancelling and replacing a defective redundant element in anintegrated circuit with another redundant element. One embodiment of thepresent invention is described as applied to a 64 megabit dynamic randomaccess memory (DRAM). It will be understood that the invention is notlimited to DRAM's, but can be equally applied to other memory devicessuch as video random access memories (VRAM) and static RAM (SRAM).Further, the present invention can be applied to any electronic circuitwhere primary and redundant circuitry is comprised of separatelyaddressable elements.

A typical 64 megabit DRAM such as those available from Micron TechnologyInc. of Boise, Id. is comprised of addressable memory cells arranged inrows and columns. The memory has eight sections with each sectioncomprised of eight sub-sections. Each sub-section having 1 megabits ofmemory arranged in 512 primary rows, or elements, and four redundantrows. Each of the rows contain 2048 addressable memory cells. Eachprimary row of the sections is uniquely addressable and any one of therows in the DRAM can be addressed using twelve address lines. If aprimary row is determined to be bad, it is known that a redundant rowcan be programmed to replace the defective row. This is accomplished byprogramming a redundant row to respond to the address of the defectiveprimary row.

Each of the four redundant rows of each sub-section has a multi-bitcomparison circuit module, or match fuse bank which is capable ofreceiving a multi-bit address signal in the form of a pre-coded signal.The match fuse bank is selectively programmable to respond to a specificaddress. Each match fuse bank evaluates the address signal and respondsif the signal corresponds to the address of a defective primary rowwhich it has been programmed to replace.

Referring to FIG. 1, one way of programming a redundant element is theuse of a match fuse bank circuit 50 corresponding to one of the 32redundant rows of one section of the DRAM. The match fuse bank circuitis comprised of six sub-banks 52 each having four address inputs 54a-d(24 total), an enable input 56, enable circuit 58, and a pre-chargeinput line 60. Each sub-bank 52 has a set of four fuses 70a-d and a setof four transistors corresponding to the four address inputs. The fusesare selectively blown to program the match fuse bank to respond to theaddress of a defective primary row. If an address received on the twentyfour address lines matches the programmed address, the match fuse bankproduces a signal on output line 72 which is used to access theassociated redundant row. This output is also used to disable theprimary rows such that the defective primary row cannot respond to theaddress signal.

Fuses 70a-d in the match fuse bank 50 are selectively blow to replace adefective primary row with an available redundant row. The fuses arepreferably laser fuses, but can be any fuse or static state maintainingdevice. By blowing three of the four fuses in each sub-bank the matchfuse bank 50 can identify the address corresponding to a defectiveprimary row of the associated section of the DRAM. That is, eachsub-bank has four possible states. By combining the six sub-banks, amatch fuse bank has 4⁶ or 4096 possible program combinations. Thesecombinations correspond to the 4096 primary rows (8 sub-sections×512primary rows) of the memory section.

In operation, enable circuit 58 which contains an enable fuse (notshown) is activated such that enable line 56 is permanently held low. Itwill be understood by those skilled in the art that any non-volatilecircuit can be used to hold enable line 56 low. Assuming for examplethat fuses 70b-d have been selectively blown in the first sub-bank 52,the source of transistor 68a is electrically connected to enable line 56through the un-blown fuse 70a. Before a match fuse bank can be addressedthe pre-charge line 60 must be activated by pulsing the line low. Thepre-charge line in turn activates p-type transistor 62 such thatsub-bank output line 74 is pulled high. Transistor 62 must be shut offprior to addressing the match fuse bank. Inverter 64 activates p-typetransistor 66 further holding line 74 high. When pre-charge line 60returns high, the inverter 64 and transistor 66 latch line 74 high. NORgate 76 produces a low output on match line 72 when any of the sub-bankoutput lines 74 are high. When an address is received by the match fusebank 50 which matches the programmed address, transistor 68a isactivated which pulls output line 74 low through non-blown fuse 70a. NORgate 76 produces a high output on match line 72 if all six sub-blocks 52receive the correct address on the four corresponding address lines54a-d. The match line is used as described above to indicate that theredundant row has been addressed and that the primary row is to bedeactivated.

It will be understood by those skilled in the art that this system canbe adapted to other memories comprising a different number of primaryelements by changing the number of fuses 70 in each sub-bank andchanging the number of sub-banks 52 in each match fuse bank 50. Further,different redundant element schemes are contemplated by the presentinvention, such as redundant columns or cells.

As detailed above, if a defective primary row was replaced or repairedwith a defective redundant row the memory was traditionally discarded.The present invention, however, provides a tool for cancelling andreplacing, or repairing, the defective redundant row with a secondredundant row. Referring to FIG. 2, a block diagram of one embodiment ofthe present invention is described. Latch match circuit 100 has a latchmatch input line 102, an enable input line 104, and feedback from matchoutput line 106. Output line 108 from the latch match circuit 100 isprovided to cancel fuse circuit 110, which in turn provides an outputline 112 to the redundant fuse match circuit 118. Fuse bank enablecircuit 114 also provides an output line 116 to the redundant fuse matchcircuit 118. Fuse match circuit 118 has a pre-charge line input 119.

Referring to FIGS. 3-5 the details of each circuit block are provided.As seen in FIG. 4, the redundant match fuse bank 120 is programmed asdescribed above for match fuse bank 50 such that output 106 is used toenable a redundant row. That is, fuses 122 are selectively blown so thataddress lines 124 and transistors 126 control the NOR gate 128 toproduce a high output on match output line 106. Line 130 must be heldlow to enable redundant match fuse block 120. This is accomplished byproviding a low signal on both line 116 and 112 so that NOR gate 132activates transistor 136. Line 112 is normally low, as described below.Line 116, however, is latched low using fuse bank enable circuit 114(shown in FIG. 5) when a redundant fuse match circuit 118 is programmed.

Referring to FIG. 5, the fuse bank enable circuit 114 comprises a staticlatch circuit comprised of a one time blowable fuse 138, inverter 140,p-type transistors 142, 144, and 146, and an input line 148. In aninactivated state (prior to enabling the redundant element), fuse 138 isintact such that the input to inverter 140 is low and the output line116 is high. As described above, redundant fuse match circuit 118 isdisabled if line 116 is high. To enable a redundant fuse match circuit,therefore, fuse 138 is blown. As a result, transistor 146 is activatedwhen input line 148 is pulsed low. The input to inverter 140 goes highthrough transistors 142 and 146 resulting in a low output on line 116.Transistor 144 is activated such that when input line 148 returns highthe fuse match enable circuit is latched low.

If after a redundant row is determined to be defective after it has beenenabled and programmed to replace a defective primary row, latch matchcircuit 100 and cancel fuse circuit 110 can be used to disable theredundant row. Referring to FIG. 3, latch match circuit 100 and cancelfuse circuit 110 are described in detail. Under normal enable conditionsline 112, as described above, must be low so that the match fuse block120 is enabled. Cancel fuse circuit 110 comprises a latch circuit 158which insures that line 112 is latched low. Latch circuit 158 isactivated by pulsing line 148 low to turn transistor 160 on, thuspulling the input to inverter 164 high and forcing output line 112 low.When line 148 returns high, output line 112 is latched low throughtransistors 162 and 163. Line 188 is normally biased at one half thesupply voltage to activate both transistor 166 and transistor 162. Itshould be noted that transistor 162 is a long L device such that using alower gate voltage on line 188 reduces the current flow through thetransistor. Program/ground line 186 is normally grounded such thatanti-fuse 170 stores a potential voltage, thereby, insuring that theinput to inverter 164 remains high. Anti-fuse 170 is a DRAM nitridecapacitor which is in essence a normal memory array cell used as ablowable capacitor. No extra process steps are therefore required in thefabrication of the DRAM to produce the anti-fuse. The anti-fuse is a onetime blowable, or programmable, element which remains shorted when thecircuit is powered down and re-powered up. It will be understood thatany non-volatile element could be used to replace the anti-fuse todisable the redundant element. For a discussion of anti-fuses see U.S.Pat. No. 5,315,177 entitled "One Time Programmable Fully-TestableProgrammable Logic Device with Zero Power and Anti-Fuse CellArchitecture" issued to Zagar et al. which is incorporated herein.

To disable or cancel a defective redundant row, latch circuit 158 mustbe set such that line 112 is permanently latched high. A five stepoperation is used to toggle and latch line 112 in a high state. Thefirst step is to shut off transistor 166 by lowering line 188. Thisisolates latch circuit 158 from the anti-fuse. The second step is topulse reset line 182 high so that any voltage stored on anti-fuse 170 isdissipated. The third step is to activate transistor 174. This steprequires that both of the inputs to NAND gate 180 go high so thatinverter 178 has a high output. While line 184 is a program cancel lineand is held high during this step, output line 108 from the latch matchcircuit 100 must be raised as detailed below. The fourth step is toraise the voltage on program/ground line 186 until it exceeds the breakdown voltage of the anti-fuse 170 such that it permanently causing ashort. The 64 megabit integrated circuit DRAM is preferably a 3.3 voltdevice. As such, a typical transistor cannot reliably handle thevoltages needed to blow the anti-fuse (around 13 volts). That is, thep-n junction break down voltage of a typical transistor is near, orbelow, the voltage needed to break down the junction of the anti-fuse.Transistors 172 and 174 are, therefore, high voltage MOS transistorsdesigned to handle relatively large potentials such that anti-fuse 170is the "weak link". These transistors are fabricated using isolationtechniques such as grounded gate, field isolation implant, or the like.It will be recognized by one skilled in the art that other fabricationtechniques could be used to effectively increase the p-n junctionbreakdown voltage above that needed for the anti-fuse.

With the anti-fuse forming a short circuit, the fifth step is to returnline 188 to its normal state so that the transistor 166 is activated andthe input to inverter 164 goes low. As a result, line 112 is latchedhigh to produce a low output from NOR gate 132. The NOR gate outputturns transistor 136 off and activates transistor 134 to de-activate thematch fuse bank 120.

The latch match circuit 100 comprises NAND gates 150, 152, and 154, andinverter 156. Prior to cancelling a redundant element, latch match inputline 102 is low and enable input line 104 is high. However, when adefective redundant row is to be de-activated, as described above, latchmatch input line 102 is raised to a high state and enable input line 104is lowered to a low state. To insure that the proper redundant row isde-activated, the defective redundant row is addressed using addresslines 124 as described above. Match output line 106 goes high and outputline 108 switches from a low state to a high state when the row isaddresses, thereby, allowing the completion of the third step asdetailed above.

FIG. 6 illustrates the key steps to cancelling a redundant element.Timing diagram line 190 illustrates the signal on line 188 which goeslow to isolate latch circuit 158 from the anti-fuse 170. Timing diagramline 192 illustrates the signal on line 182 which is pulsed high afterline 188 goes low to discharge the anti-fuse. Match line 106 and line184, illustrated by timing diagram lines 194 and 196, respectively, areboth raised to activate transistor 174. Timing diagram line 198 showsthe program/ground line 186 which is then raised to blow the anti-fuse.The timing diagram is not to scale and should not be taken as anindication of the exact voltage level on the individual lines. After theanti-fuse is blown, the program/ground line 186 returns low. As detailedabove, the input to inverter 164 is connected to ground through theshorted anti-fuse 170 to permanently disable the match fuse bank 120.

Once the anti-fuse 170 is blown, the defective match fuse bank 120 ispermanently disabled and cannot be addressed by address lines 124. Asecond redundant row can, therefore, be activated and programmed torespond to the address of the defective primary row, as detailed aboveusing the match fuse block.

SUMMARY

The present invention provides a circuit and method for cancelling aredundant element of an integrated circuit. This invention can beprovided in any integrated circuit having addressable primary elementsand selectively addressable redundant elements. The invention provides acircuit and method for permanently disabling a defective redundantelement which was previously enabled. A second redundant element,therefore, can be used to replace the defective element.

What is claimed is:
 1. A method of repairing an integrated circuit, themethod comprising the steps of;programming a first redundant element torespond to an address of a defective selectively addressable primaryelement; one time enabling the first redundant element; permanentlydisabling the first redundant element after the step of one timeenabling the first redundant element; programming a second redundantelement to respond to an address of the defective selectivelyaddressable primary element; and one time enabling the second redundantelement.
 2. The method of claim 1 wherein the integrated circuit is adynamic random access memory (DRAM) and the first and second redundantelements, and the primary element are memory elements.
 3. The method ofclaim 1 wherein the step of programming a first redundant elementcomprises the step of selectively blowing a fuse circuit in a fuse bankenable circuit.
 4. The method of claim 1 wherein the step of permanentlydisabling the first redundant element comprises the step of selectivelyblowing a fuse circuit in a cancel fuse circuit.